Ferroelectric properties of niobium-doped strontium bismuth tantalate films

The characteristics of ferroelectric thin films of strontium bismuth tantalate (SBT) and niobium-doped strontium bismuth tantalate (SBTN) deposited by radio-frequency (RF) magnetron sputtering on Pt/TiO₂/SiO₂/Si substrates were investigated. For the formation of the structure of the ferroelectric material, the deposited films were subjected to a subsequent annealing at temperatures of 970–1070 K in an O₂ atmosphere. The results of the X-ray diffraction analysis demonstrated that, in contrast to SBT films, in which the Aurivillius phase is formed only at annealing temperatures of 1050–1070 K, the formation of this phase in SBTN films is observed already at a temperature of 970 K. The dependences of the dielectric permittivity, remanent polarization, and coercive force of the SBT and SBTN films on the subsequent annealing conditions were determined. It was found that, upon doping of the SBT films with niobium, the remanent polarization increases by a factor of approximately three, the Curie temperature increases by 50 K, and the dielectric permittivity also increases. It was revealed that, in contrast to the SBT films, the polarization of the SBTN films is observed already at an annealing temperature of approximately 970 K. It was shown that the replacement of SBT films by SBTN films in the manufacture of high-density nonvolatile ferroelectric randomaccess memory (FeRAM) capacitor modules makes it possible to decrease the synthesis temperature from 1070 to 990–1000 K, which improves the compatibility with the planar technology of semiconductor devices. However, it turned out that an increase in the coercive field makes niobium-doped SBT films less attractive for the use in FeRAM.     

Preparation and properties of lead titanate thin ferroelectric films

A new technique is proposed for preparing lead titanate ferroelectric films. The technique involves a solid-phase reaction in an oxygen environment between titanium and lead layers deposited on a substrate. The thicknesses of the component films being deposited are chosen based on the stoichiometric ratio in the compound to be synthesized. The composition and structure of the film obtained are checked by x-ray phase analysis. The films exhibit a dielectric hysteresis loop and a temperature dependence of the permittivity characteristic of ferroelectrics. A study has been made of the temperature and thickness dependences of the film coercive field. They also are shown to follow a pattern typical of ferroelectrics.     

Preparation and optical properties of barium titanate thin films

Barium titanate (BTO) films were prepared by sol-gel spin-coating technique. The crystal structure and optical properties of BTO films have been investigated. The results indicate that the BTO films are single perovskite phase having tetragonal symmetry. The band gap of the BTO films increases with the increasing of layer number and decreasing of solution concentration. The transmittance and band gap of the BTO films annealed at 900 °C is more than that of the BTO films annealed at 700 °C when wavelength is 200-1000 nm. When wavelength is 400-1000 nm, the absorption coefficient α obtained by experiment is higher than that obtained by calculation (close to zero).     

Dielectric properties of lead titanate thin films on corundum substrates

Ferroelectric films of lead titanate were synthesized on corundum (Al₂O₃) substrates. The main parameters that characterize the dielectric hysteresis, such as the spontaneous polarization, coercive force, internal bias field, and the phase-transition temperature, were determined. The dispersion of the permittivity was studied using impedance spectroscopy in wide temperature and frequency ranges. An anomaly in the frequency dependence of the complex permittivity was revealed at frequencies of the order of 10⁷ Hz.     

Electrical properties of thin lead titanate films on polycor substrates

The dielectric properties of synthesized films, including the permittivity dispersion, have been investigated. The fatigue processes in the films have been experimentally studied as a function of the number of cycles of external voltage switching. It is established that the main dielectric characteristics are partially recovered in the absence of external actions.     

Enhanced ferroelectric properties of vanadium doped bismuth titanate (BTV) thin films grown by pulsed laser ablation technique

Bi_3.99Ti_2.97V_0.03O₁₂ (BTV) thin films were grown by pulsed laser deposition at substrate temperatures ranging between 650 and 750 °C. The structural phase, and orientation of the deposited films were investigated in order to understand the effect of the deposition parameters on the properties of the BTV films. As the substrate temperature was increased to 700 °C, the films started showing a tendency of assuming a c-axis preferred orientation, while at lower temperatures polycrystalline films were formed. The Au/BTV/Pt capacitor showed an interesting dependence of the remnant polarization (P_r) as well as dc leakage current values on the growth temperature. The film deposited at 675 °C showed a very large 2P_r of 42 μC cm⁻², which is the largest for BTV thin films among the values reported so far.     

Transparent,photocatalytic, titania thin films formed at low temperature

A convenient method for the preparation of transparent, photocatalytic titania thin films is described. The films do not require annealing or thermal processing to develop photoactivity, thus can be applied to many thermally-sensitive substrates. Oxalic acid is used in place of the usual mineral acids to peptize the precipitated hydrous titania formed from the hydrolysis of titanium iso-propoxide. This leaves no inorganic residues in the film resulting in a higher quality film. The mineral phase and the photocatalytic activity produced are strongly influenced by the ratio of oxalic acid:titanium iso-propoxide employed. The peptization is carried out at 65 °C with vigorous stirring for 1 h in water containing 15% v/v ethanol, followed by a hydrothermal step at 95 °C. High oxalic acid:Ti molar ratios (0.5:1) result in rutile free sols, while lower ratios (0.25:1) result in anatase, rutile, brookite and TiO₂(B) in varying proportions. The films were exposed to low level UV light to cure, and photodecompose the residual organic components in the film. The photodecomposition of residual oxalic acid in the cast films were monitored using infrared spectroscopy. Photo-activity of the UV-cured films was compared by monitoring the decoloration of methylene blue stains on the film, by UV–Vis spectroscopy. Transmission was greater than 99% across the visible light region (400–800 nm).     

Optical and electrical properties of thermally oxidized bismuth thin films

Bismuth trioxide (Bi₂O₃) thin films were prepared by dry thermal oxidation of metallic bismuth films deposited by vacuum evaporation. The oxidation process of Bi films consists of a heating from the room temperature to an oxidation temperature (T_o = 673 K), with a temperature rate of 8 K/min; an annealing for 1 h at oxidation temperature and, finally, a cooling to room temperature. The optical transmission and reflection spectra of the films were studied in spectral domains ranged between 300 nm and 1700 nm, for the transmission coefficient, and between 380 nm and 1050 nm for the reflection coefficient, respectively. The thin-film surface structures of the metal/oxide/metal type were used for the study of the static current-voltage (I-U) characteristics. The temperature of the substrate during bismuth deposition strongly influences both the optical and the electrical properties of the oxidized films. For lower values of intensity of electric field (ξ < 5 × 10⁴V/cm), I-U characteristics are ohmic.     

Nonlinear optical properties of thin films at a low light intensity

Nonlinearity of optical parameters of semiconductor and dielectric films for a 0.63-nm-wavelength light with intensity lower than 100 mW/cm² is discovered and studied.     

Effects of deposition temperature and thickness on the structural properties of thermal evaporated bismuth thin films

Bismuth (Bi) thin films of different thicknesses were deposited onto Si(1 0 0) substrate at various substrate temperatures by thermal evaporation technique. Influences of thickness and deposition temperature on the film morphologies, microstructure, and topographies were investigated. A columnar growth of hexahedron-like grains with bimodal particle size distribution was observed at high deposition temperature. The columnar growth and the presence of large grains induce the Bi films to have large surface roughness as evidenced by atomic force microscopy (AFM). The dependence of the crystalline orientation on the substrate temperature was analyzed by X-ray diffraction (XRD), which shows that the Bi films have completely randomly oriented polycrystalline structure with a rhombohedral phase at high deposition temperature (200 °C) and were strongly textured with preferred orientation at low deposition temperatures (30 and 100 °C).     

退火温度对低温生长MgxZn1-xO薄膜光学性质的影响

用射频磁控溅射法在80℃衬底温度下制备出MgxZn1-xO(x=0.16)薄膜,用X射线衍射(XRD)、光致发光(PL)和透射谱研究了退火温度对MgxZn1-xO薄膜结构和光学性质的影响.测量结果显示,MgxZn1-xO薄膜为单相六角纤锌矿结构,并且具有沿c轴的择优取向;随着退火温度的升高,(002)XRD峰强度、平均晶粒尺寸和紫外PL峰强度增大,(002)XRD峰半高宽(FWHM)减小.结果证明,用射频磁控溅射法通过适当控制退火温度可得到高质量MgxZn1-xO薄膜.     

Effects of annealing conditions on the electrical properties of Bi4-xNdxTi3O12 (x = 0.46) thin films processed at low temperature

Neodymium-substituted Bi₄Ti₃O₁₂ (BNdT) thin films were prepared by a chemical solution deposition technique on platinum- coated silicon substrates. All of the samples were annealed at the relatively low temperature of 600 °C by a rapid thermal annealing process in different atmospheres, such as O₂, air, and nitrogen, and vacuum. Irrespective of different annealing atmospheres, all of the BNdT thin films exhibit good ferroelectric properties, such as a saturated hysteresis loop, good fatigue endurance, and low leakage current density. A large remanent polarization (P_r) of ∼48 μC/cm² with an electric field of 240 kV/cm was observed from the BNdT thin film annealed in O₂ atmosphere. The BNdT thin films annealed in nitrogen and vacuum, at reduced oxygen partial pressures, exhibit smaller P_r than that annealed in oxygen. The difference of P_r of the BNdT thin films annealed in different atmospheres may originate from differences in the grain sizes and the number of oxygen vacancies. PACS 77.55.+f; 77.80.-e; 77.80.Fm; 81.15.-z     

Fatigue of lead titanate and lead zirconate titanate thin films

The fatigue of lead titanate and lead zirconate titanate ferroelectric thin films, i.e., a change in the polarization as a function of the number of switching cycles in an external electric field, is investigated experimentally. The threshold numbers of switching cycles are determined to be 10¹⁰–10¹¹ for the lead titanate films and 10⁹–10¹⁰ for the lead zirconate titanate films. It is shown that a change in the temperature does not substantially affect the threshold number of switching cycles at which the switched polarization decreases drastically. However, an increase in the external field strength leads to a noticeable decrease in the threshold number of switching cycles. The process of fatigue is accompanied by an increase in the coercive field and the internal bias field. It is established that, as the number of switching cycles increases, the internal bias field changes more significantly as compared to the coercive field. The absence of a change in the phase composition in repeatedly switched samples indicates that the fatigue processes have a nonchemical nature. The anomaly observed in the frequency dependence of the permittivity in the frequency range 10⁶–10⁷ Hz due to the domain structure disappears after multiple switching cycles. This suggests that the observed fatigue phenomenon has a domain nature.     

Mechanical properties of sol-gel derived lead zirconate titanate thin films by nanoindentation

Lead zirconate titanate (PZT) thin films are deposited on platinized silicon substrate by sol-gel process. The crystal structure and surface morphology of PZT thin films are characterized by X-ray diffraction and atomic force microscopy. Depth-sensing nanoindentation system is used to measure mechanical characteristics of PZT thin films. X-ray diffraction analyses confirm the single-phase perovskite structures of all PZT thin films. Nanoindentation measurements reveal that the indentation modulus and hardness of PZT thin films are related with the grain size and crystalline orientation. The increases of the indentation modulus and hardness with grain size are observed, indicating the reverse Hall-Petch effect. Furthermore, the indentation modulus of (1 1 1)-oriented PZT thin film is higher than those of (1 0 0)- and random-oriented films. The consistency between experimental data and numerical results of the effective indentation moduli for fiber-textured PZT thin films using Voigt-Reuss-Hill model is obtained.     

Oxidation temperature dependent optical properties of bismuth oxide thin films: Effect of vapour chopping and air exposure

The optical properties of vapour chopped and nonchopped bismuth oxide thin films of two thicknesses 1500 Å and 2000 Å have been studied. The films were prepared by thermal oxidation in air; of vacuum evaporated vapour chopped and nonchopped bismuth thin films. As revealed by XRD studies, multiphase and polycrystalline bismuth oxide thin films were obtained. The refractive index was found to increase with the thickness and exposure to air for 40 days. The vapour chopped films showed higher refractive index, band gap and lower grain size than those of nonchopped films. The films showed high transmittance in the visible spectrum. The ageing effect on the vapour chopped films was found low.     

热处理对TbDyFe薄膜性能的影响

采用磁控溅射方法利用镶嵌靶材制备非晶TbDyFe薄膜,然后分别在150,300,450,550℃下退火,研究了热处理对TbDyFe薄膜性能的影响.结果显示,550℃温度下退火的样品出现RFe2相.随着退火温度的升高,晶粒逐渐长大,空洞网络变短而宽.薄膜的磁特性显示,其矫顽力及矩磁比均在出现RFe2相的550℃退火的样品中达到最大值.随着退火温度的增加,薄膜的易磁化轴从垂直膜面方向向平行膜面方向转动.     

Nanomagnetism in nanocrystalline multiferroic bismuth ferrite lead titanate films

Multiferroics conventionally refer to the materials exhibiting co-existing electric, magnetic, and structure order parameters. Interplay between ferroelectricity, magnetism, and ferroelasticity in a single phase makes multiferroics truly multifunctional providing control over magnetic and electric ordering by applying electric and magnetic fields, respectively. Incorporation of multiferroic-based components into nanoscale applications will enable additional degrees of freedom in manipulating with spin and charge not easily attainable otherwise. Multiferroic bismuth ferrite lead titanate has been chemically synthesized in form of nanocrystalline films. The morphology of the films revealed a single perovskite phase confined within crystalline grains of few tens of nm in size. The films were found to exhibit ferroelectricity and ferromagnetism with characteristic electric polarization and magnetization hysteresis loops, transformations associated with spin reorientation in an external magnetic field and the spin-glassy behavior well above the room temperature. High degree of magnetic frustration and disorder in the spin system spatially confined in the nanograins, distribution of the grains anisotropy axis, inter-grain interactions, and the effects of uncompensated spins on the large effective surface/interface favored by the nanocrystalline morphology were assumed to be responsible for the anomalous magnetic properties and glassy dynamics in the films.